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Branton Campbell in the lab

Branton J. Campbell

Professor

Department of Physics & Astronomy

Brigham Young University

Provo, UT 84602, USA

Tel: 801-422-5758, Fax: 801-422-0553

Email: branton_campbell[at]byu.edu

//physics.byu.edu/faculty/campbell/

Research Interests

I apply state-of-the-art x-ray and neutron scattering techniques to study local and long-range structures in a variety of complex solids, including fast-ion conductors, ferroelectric relaxors, high-temperature superconductors, and colossal magnetoresistive manganites, where nanoscale structural features influence macroscopic physical properties. This includes the development of symmetry-mode analysis (through the tools of the ISOTROPY Software Suite) for the determination, refinement and interpretation of distorted structures involving lattice strains, atomic displacements, magnetic moments and occupational orderings at both commensurate and incommensurate wavevectors.

Superspace-Group Equivalence

To uniquely identify the superspace-group (SSG) symmetry of a modulated crystal, one needs a way to test the equivalence of two distinct sets of superspace-group operators. A highly efficient and robust algorithm, which was recently developed for tabulating SSGs, has now been employed to identify the symmetries of each of the (3+2)D and (3+3)D modulated structures published in the literature to date.

Selected Publications

The cation-vacancy ordering transition in dehydrated Na₆ sodalite

Branton J. Campbell (et al.)

Variable-temperature synchrotron x-ray powder diffraction data from dehydrated Na-6[Al6Si6O24] reveal a structural phase transition that involves both the commensurate ordering of the extra-framework Na cations and a one-dimensional incommensurate modulation of the framework. Peak splittings and superlattice reflections implicate an orthorhombically-distorted volume-doubled supercell at room temperature (a=12.9432 Angstrom, b=12.8403 Angstrom, and c=9.1372 Angstrom) similar to(root(2) over bara x root(2) over bar axa). These data also included additional superlattice peaks associated with an incommensurate long-period modulation with tau=(3/2 1/2 1)/8.9. All unique orderings of the 12 Na cations among the 16 available sites of the commensurate supercell are enumerated. A unique solution is identified that minimizes the structural energy and accounts for the observed superlattice peaks. Rietveld analysis reveals a significant Pauling "partial-collapse" tilt angle in the cation-ordered phase, that appears to decrease with increasing temperature. (C) 2000 American Institute of Physics. [S0021-9606(00)00844-8].

Charge ordering and phase competition in the layered perovskite LaSr₂Mn₂O₇

B. J. Campbell (et al.)

Charge-lattice fluctuations are observed in the layered perovskite manganite LaSr2Mn2O7 by Raman spectroscopy at temperatures as high as 340 K, and with decreasing temperature they become static, forming a charge-ordered (CO) phase below T-CO = 210 K. In the static regime, superlattice reflections are observed by neutron and x-ray diffraction with a propagation vector (1/4, - 1/4, 0). Crystallographic analysis of the CO state demonstrates that the degree of charge and orbital ordering in this manganite is weaker than that in the three-dimensional perovskite manganites. Below T-N= 170 K, type-ii antiferromagnetism (AF) develops and competes with the charge ordering, causing it to eventually melt below T* = 100 K. High-resolution diffraction measurements suggest that the CO and AF states do not coincide within the same region of material, but rather coexist as separate phases. The transition to typed antiferromagnetism at lower temperatures is characterized by the competition between these two phases.

The synthesis of zeolite ERS-7 and its structure determination using simulated annealing and synchrotron X-ray powder diffraction data

B. J. Campbell (et al.)

ERS-7, a small-pore zeolite with a novel framework topology, was synthesized hydrothermally using N,N-dimethylpiperidinium hydroxide as a structure directing agent. Its structure was determined by using simulated annealing and refined from synchrotron X-ray powder diffraction data. It consists of 17-sided “picnic basket”-shaped cages connected by 8-membered-ring windows with 4.7 ´ 3.5 Å free dimensions. The calculated lattice energy and observed framework density are consistent with the empirical linear relationship obeyed by other zeolite frameworks.

The synthesis of the new zeolite, ERS-7, and the determination of its structure by simulated annealing and synchrotron X-ray powder diffraction

B. J. Campbell (et al.)

The new zeolite ERS-7 (EniRicerche-molecular-Sieve-7), which was synthesized within a narrow temperature and compositional range using N,N-dimethylpiperidinium as a structure directing agent, has a structure consisting of 17-sided (4(6)5(4)6(5)8(2)) picnic-basket-shaped cages connected by 8-membered ring windows.

NMR study of ammonium magnesium langbeinite

Branton J. Campbell, Harold T. Stokes, and Juliana Boerio-Goates

Recent studies have shown that ammonium langbeinites have low-temperature phase transitions uncharacteristic of the rest of the langbeinite family. In this study, proton NMR spin-lattice relaxation times T1 and spin-spin relaxation times T2 for powder samples of (NH4)2Mg2(SO4)3 are reported in the temperature range from 77 to 373 K. The ammonium ions occupy two different kinds of sites in the crystal. The measured barrier to thermally activated ammonium reorientations is found to be 2.0 and 3.3 kcal/mol for the two sites, respectively. Additionally, there is no observable discontinuity in T1 at the reported phase transition at 161 K, in agreement with heat-capacity studies, suggesting that the transition mechanism may be different from those of most langbeinites.